Several negative regulators of angiogenesis are the same proteins that control the formation and dissolution of fibrin clots during wound healing. These include coagulation proteins antithrombin III, prothrombin and fibrinogen and their proteolytic fragments, prothrombin fragments-1 and -2 and angiostatin. Beta-2-glycoprotein 1 (beta2GBP1), a 50 kDa plasma protein, regulates thrombosis by competing for the assembly of coagulation factors. Using a subcutaneous implant comprised of gelatin and agarose that accurately quantifies the formation of new blood vessels, we observed that embedded beta2GBP1 completely blocked the invasion of new blood vessels into the implant suggesting that it could inhibit tumor angiogenesis. Therapy studies using the B16 melanoma, UV2237 flbrosarcoma and TRAMP prostate carcinoma mouse tumor models showed that repeated administration of the protein significantly inhibited the growth of all the tumors studied and lung metastasis in the B16 melanoma model. In vitro studies showed that beta2GBP1 specifically inhibited endothelial cell growth, cord formation and cell migration, suggesting that inhibition of tumor growth might be due to inhibition of angiogenesis. Studies on the effects of the protein on normal vasculature in vivo, on the other hand, showed complete inhibition of growth factor-induced, but not chemically-induced blood vessel dilatation suggesting that inhibition of tumor growth occurred via an angioectasia-dependent mechanism. In this proposal, we will examine the mechanism of beta2GBP1-dependent inhibition of tumor cell growth and its potential as a new therapeutic modality for the treatment of cancer.